Abstract: A data processing system, a data retrieval system, a data processing method, and a data retrieval method requiring less time to update data to be used in retrieval. The dictionary data generator generates dictionary data from a code string on which data retrieval is to be performed using a key string. The dictionary tree generator generates a dictionary tree based on the dictionary data. The text data generator generates text data based on the code string. The text tree generator generates a text tree based on the text data. The retrieving unit retrieves a trailing string whose leading string contains part of or the whole key string, based on the dictionary tree 4 and the text tree. The changing unit changes the code string, and the updating unit updates the dictionary data, dictionary tree, text data, and text tree of the code string associated with the change, based on the contents of the change.